1. Field of the Invention
This invention relates to leather treatment products.
2. Description of the Art Practices
The leather industry is itself nearly as old as mankind, it being found that animal skins when properly treated make excellent long lasting clothing items as well as other products. The green hides obtained from the slaughterhouse are usually treated by removing the hair from the hide. This is referred to as the hair burn operation. A secondary step in the treatment of the hide after the hair is removed is the extraction of the essential oils present in the hide leaving the protein component of the hide intact. Thereafter protein coagulating agents such as chromium are used to cross-link the protein thereby preserving the leather.
The essential oils of the hide are then returned into the product to provide a flexible product. The oil replacement step, depending on the type of leather being treated, may be with a different formulation of oils than that originally removed from the hide. That is, components such as soaps and other unwanted by-products are conventionally removed by a leather chemical processor before the oil is replaced in the leather. At this point the product is referred to as the leather crust which is ready for the various finishing operations conducted by the leather industry.
The product is then treated with a dye coat or stain coat which may be sprayed or seasoned onto the product. Typically the dyes used are of the aniline family which are both organic and transparent materials which provide a deep luster and color in the product. The term seasoning may be equated with swabbing on and hand treating of the leather as opposed to the spray. Depending on the type of garment or leather product involved one or both methods may be used.
The next step after placing the dye coat or stain coat (to give the deep coloration into the leather) is the pigmentation step. Typically the pigmentation step involves applying to the product a mixture of an inorganic pigment, resins, waxes, binders and water. The water is of course used mainly to obtain the consistency of the product to be applied and should not be considered to be an essential ingredient. Typical pigments used to color the leather are: titanium dioxide, lead chromates, lead molybdates, iron oxides, zinc sulfides and cadmium sulfides, selenides and sulfoselenides.
The resins employed at this point are materials such as acrylic resins or latex materials which hold the coating containing the pigment to the product and also to allow flexibility so that the pigmentation is not free to crack or flake off of the product. The waxes which are applied at this point are typically wax emulsions which add hardness and feel (hand) to the product. The concept of feel or hand is subjective, however, it can commonly be observed that some garments feel good while other garments do not. Typically plastic products such as those formulated from vinyl resins do not breathe and have an extremely smooth finish. Leather while having a smooth feeling finish should have a sufficient degree of resistance to the stroke of the hand to give the feeling that one is dealing with a natural skin. The waxes employed at this point allow a plate release to be effected upon sheet pressing and to mold the smooth finish as previously described. The plate release arises from the fact that many leathers are treated by pressing with a hot plate which is typically a smooth metal surface either as a roller or a flat plate. It is of course essential that the wax assists in obtaining a product where the leather treatment chemicals are not retained on the plate but rather are forced into the leather product.
The binders employed at this point are ordinarily casein based which assists in drying the film to a stable surface. That is even though the plate release may not be a problem because of the wax emulsion it is generally found necessary to utilize a binding agent to ensure that the product does not become freed from the leather. As previously mentioned, the water is used as a carrier and is not of any particular consequence.
The particular aspect with which the present invention is concerned is in the top coat operation. Top coating is the last application or applications applied to the leather. That is, after the pigmentation step has been completed, a top coat of one or two more coats is added to ensure that there is a flexible coating over the pigmentation to prevent wear and to add a degree of hardness to the coating beyond the natural strength of the leather. Of course the top coating should not be such that it leaves the goods with a plastic feel or as previously mentioned a lack of hand or feel.
The conventional top coats known in the art are those containing urethanes, vinyl coatings, cellulose acetate butyrate resins (CAB) and nitrocellulose. Each of the foregoing types of top coatings have particular applications.
The urethane top coats are exceptional with respect to wear and tear. Automobile seats where there is considerable wear are often protected by a top coat which is urethane based. Urethane unfortunately does not breathe and does not exhibit a reasonable feel such as is desired in a leather product. However, for the wear factor the leather is quite well protected by a urethane coating. The lack of breathing however can cause the product to become quite hot and this phenomena will be readily recognizable to those who have sat upon a urethane coated automobile seat.
The vinyl coatings also show promise in resistance and are typically utilized for the top coating of childrens' shoes. Automobile seats and leather furniture are also often treated with vinyl. Again the vinyl exhibits a plastic feel rather than the natural hand of other types of leather, and vinyl does not breathe and becomes hot when it is in close contact with the human body.
The cellulose acetate butyrate resins are a first attempt at reducing the need for nitrocellulose coatings as described below. The CAB resins are non-yellowing which is a stated defect in nitrocellulose treated products. The feel of CAB products is good but is at a substantial cost when compared to nitrocellulose. Accordingly the cellulose acetate butyrate coatings have found limited utility due to the high cost.
Nitrocellulose top coats are the standard in the industry particularly for shoe upper leather. That is, the general requirements for shoe upper leather are that the coating be breathable, have a high degree of resistance to scuffing and also present a highly polished surface. As previously noted, the nitrocellulose top coats are deficient in that they have limited utility on white and pastel colors in that ultraviolet radiation yellows nitrocellulose top coated products. Accordingly there is a limiting factor in nitrocellulose usage. A second deficiency in utilizing nitrocellulose is that it must be used in a solvent (organic) or solvent plus water borne formulation. The difficulties in utilizing nitrocellulose are that the product is flammable and can be flammable on the treated goods even after the solvent is removed. It may be remembered that nitrocellulose is also referred to as gun cotton and the products must be stored and mixed under careful conditions to avoid the possibility of fire. Nitrocellulose as noted is the standard in the industry, as it provides shoe upper leather with good hand and resistance, non-plastic feel and also protects the pigmented leather beneath the top coat.
One attempt which has been made to avoid the flammable nature of the nitrocellulose is to include a chlorinated solvent which functions to lessen the explosive nature of the vapors of the nitrocellulose and the organic solvents otherwise employed in the formulation. This solution presents other problems including air polution due to these materials and worker exposure. It is also noted that nitrocellulose is difficult to work with in that once the solent is removed (or should the nitrocellulose be dumped into a drain) it is not water soluble and exhibits a gelling tendency. This of course may block the sewer drains or provide difficulty in removing the nitrocellulose from the container in which it is stored.
The last aspect of the top coating operation is to plate the leather article. The methods of plating include pressing the top coated leather article with a hot plate or a hot roll such as the Finiflex roll machine. The Finiflex roll machine, as the name implies, rolls over the leather and raises the temperature of the leather to a temperature to about 140.degree. C. to about 200.degree. C. The top coat must be resistant to removal as was previously noted in the discussion above during the plating operation.
It therefore remains to find a suitable top coat to replace nitrocellulose. The desirable aspects of such a top coat are to be an aqueous borne material which does not yellow but gives good hand and protects from scuffing. The present invention has solved the problem of finding a replacement for the nitrocellulose based coatings in a concentrated aqueous formulation.
Throughout the specification and claims percentages and ratios are given by weight, temperatures are in degrees Celsius and pressure is expressed in atmospheres. A vinyl copolymer is defined as a polymer formed from monomers having unsaturation which becomes saturated and during linear polymerization.